It can't be helped, there is no way to auto check for spam.
Just post an announcement to your board with an excerpt from the spam that you will not tolerate anyone posting it anymore.
BTW its illegal anyway. You can turn that poster in to the USPS and FEd Govt since its illegal.
Just put your foot down and say that you will suspend the person who posted it for 1 week/month/year if they do it.
Here is more information you can post with your warning:
http://www.ftc.gov/bcp/conline/pubs/alerts/doznalrt.htm
http://www.ftc.gov/bcp/conline/edcam...ail/index.html
http://www.ftc.gov/opa/2002/02/eileenspam1.htm
http://www.scambusters.org/legends.html
http://www.scamfreezone.com/scams.shtml (which shows yu the variations of the scam you have)
And that any posting shall be reported to:
http://www.ftc.gov/bcp/conline/edcam...l/reportit.htm